It is generally known to use polytetrafluoroethylene (PTFE) aqueous dispersions in order to obtain coatings exhibiting high stabilities to heat and to chemical agents with good properties of mechanical resistance to wear and to friction. It is known as well to use PTFE modified with little amounts of a comonomer, which in most cases is fluorinated, as well as to use mixtures of PTFE with other fluorinated polymers, in order to obtain improved characteristics of imperviousness and flexibility of the coating layer. The essential technical problem in the production of these coatings consists in the adhesion to the metal substrate, such adhesion having to be very high, also in consideration of the temperatures, often very high, which the articles are subjected to in use.
A good adhesion is attainable with the presence of chromic acid in the polymer aqueous dispersion, in particular in combination with phosphoric acid. This system is not allowable if the treated articles are utilized in the foodstuffs sector, due to the high toxicity of the chromic acid, which is present, although only in traces, in the obtained polymeric coating layer.
From U.S. Pat. No. 3,634,353 it is known how to use monobasic phosphates of divalent cations in combination with phosphoric acid for the preparation of coating compositions.
In this patent it is of substantial importance, in order to obtain a good adhesion, that the molar ratio: cations/PO.sub.4.sup.-- should be &lt;0.5.
In particular, calcium is designated as the preferred cation. Tests carried out by the Applicat hereof have proved that this limit is critical in order to obtain good adhesion (See example 1A on pages 10, 11 and 12).